FACTORS AFFECTING DEVELOPMENT OF MUD-CRACKS 137 



amount of powdered marl was added to the second (2b), while the 

 third lot was left a clay and water mixture (2c). These three mix- 

 tures, representing sandy mud, marly mud, and clay mud, were 

 placed in three shallow pans for desiccation in the sun. The result- 

 ing mud-cracks show that the clay mud (2c) cracked into much 

 larger polygons (Fig. 1) than either the sandy or marly mud (Fig. 2) . 

 The sandy mud (3a) developed more than three times as many 



Fig. 1. — Normal fresh-water mud-cracks in blue-clay mud. f natural size 



polygons (Fig. 3) as the clay mud, while the marly mud showed 

 more than twice as many polygons as the clay mixture. The large 

 number and very angular course of many of the mud-cracks in 

 the sandy mixture are characteristic features which distinguish 

 this mixture from either of the other two. 



Experiment 3. — A portion of the same clay mixture used in 

 experiment 1 was placed in a shallow pan n inches in diameter. 

 The water was allowed to evaporate slowly without exposure to 



